Influencing factors of rainwater transformation and soil erosion in thin soil hillslope of rock desertification regions

Abstract: Karst landforms are characterized by surface-underground double hydrological structure, due to strong karstification. There are very complex rainwater transformations and soil erosion under the double hydrological structure. It is a high demand to clarify the influencing factors of rainwater transformation and soil erosion on karst hillslopes. In this study, a rainfall simulation was performed on karst hillslope to explore the runoff and soil erosion at the surface and below ground under different rainfall intensities, slope gradients, and rainfall times. Among them, the rainfall intensities were 50, 75, and 100 mm/h; while the rainfall time was successive three ones at 24 h interval under similar rainfall intensity (1st, 2nd, and 3rd); the slope gradient were 5°, 15°, and 25°; two types of hillslope were with/without the fissure on underground bedrock (No Underground Fissure Flow (NUF) and Underground Fissure Flow (UFF)). Two hydrological situations were selected, including two soil boxes with/without fissures at the bottom. The length, width, and height of the two soil boxes were 1, 0.5, and 0.15 m, respectively. The Surface Runoff (SR) and Subsurface Runoff (SBR) of two hillslopes treatment were collected through the experimental soil boxes, together with the Soil Rock Interface Flow (SRIF) of NUF and UFF. The results showed that 1) the SR rate increased with the increase of rainfall intensity, slope gradient, and rainfall time; a similar trend was also found in the SR rate under different rainfall intensity and slope gradient. But the SBR rate of the two hillslopes decreased with the increase in rainfall time. The NUF and UFF rate of soil rock interface increased firstly, and then decreased, with the increase of rainfall intensity, whereas, decreased with the increase of slope gradient and rainfall time. 2) The soil-rock interface flow (24%-39% of the total rainwater) of NUF and UFF (28%-51% of the total rainwater) dominated the runoff process of karst hillslopes, where there were small rainfall intensity (50 and 75 mm/h), slope gradient (5°and 15°) and rainfall time (First rainfall). However, the SR (30%-50% of total rainwater and 25%-43% of total rainwater) dominated the runoff processes of two treatment hillslopes with the increase of factors. 3) There was also an important role of drive factor in the rainfall intensity (The NUF and UFF regression coefficient: 0.61 and 0.63, (P<0.01), the same as the slope gradient (The NUF and UFF regression coefficient: 0.38 and 0.38) during surface soil erosion process of two hillslopes treatment (P<0.01). There was a very slight effect of rainfall intensity, slope gradient, and rainfall time on the soil erosion below the ground of the two hillslopes treatment. The R2 of the regression equation was also less than 0.3, indicating no significant (P>0.05) effect of rainfall intensity, slope gradient, and rainfall time on soil erosion below ground. Overall, the surface soil erosion dominated the soil erosion process of NUF and UFF, where the ratio of surface soil erosion to total soil erosion were 54%-97% and 39%-96%, respectively. The soil-rock interface erosion or underground fissure erosion was the secondary erosion process (1%-45% or 2%-60%). These findings can provide the basic information for a better understanding of the dive factors of rainwater transformation and soil erosion on karst hillslopes.